This invention relates generally to magnetic resonance imaging (MRI), and more particularly to method and apparatus for spatial encoding in MRI applications.
In magnetic resonance imaging, spatial localization is achieved using selective excitation or Fourier encoding. These two approaches have rather different, sometimes complementary, characteristics. Selective excitation is realized by applying a band-limited RF pulse in the presence of a slice selection gradient. The spatial resolution attainable with this technique is determined by the slice profile which is a direct function of the slice selection gradient strength and the shape of the RF pulse. In forming an image based on selective excitation (e.g., line scanning), data are acquired using multiple localized excitations each affecting a different region of the imaged field of view. Hence, RF excitations can be interleaved withing the same repetition time, TR, for added efficiency. Moreover, it is possible to update the image locally with few excitations to follow dynamic events occurring in a portion of the field of view. Nevertheless, since each region is excited separately, the signal-to-noise ratio (SNR) can be rather low since no data averaging is implicitly incorporated in this process.
On the other hand, Fourier encoding achieves spatial localization through spatial frequency or phase encoding while the entire volume of interest is excited. By applying varying amounts of Fourier encoding (phase encoding steps) to sufficiently sample the k-space region of interest, the field of view can be reconstructed using a Fourier transform operation. This means that the resolution in this case is in principle unrestricted while keeping an excellent SNR as a direct result of the implicit averaging in the Fourier transform operation. Since the entire field of view is excited each time, the technique is however susceptible to partial saturation as a function of the time interval between excitations. Moreover, the global nature of Fourier encoding lacks the desirable spatial localization property, making it necessary to re-acquire the entire data set to update any portion of the image.
The differences between selective excitation and Fourier encoding are well-recognized and have been carefully taken into consideration in various applications. For example, in volume imaging, multi-slice imaging takes advantage of the interleaving capability of selective excitation to efficiently collect images with high contrast, while 3-D acquisition is routinely used to obtain high resolution, high SNR images. In certain applications, the use of either one of these two techniques may not be optimal and a combination of the two methods may in fact be desired. For example, in time-of-flight magnetic resonance angiography (TOF MRA), the trade-off between SNR, slice thickness, and contrast makes it suboptimal to use either multi-slice or 3-D acquisition. This motivates the introduction of hybrid techniques to combine features from both techniques. For example, in multiple overlapping thin slab acquisition (MOTSA) (See: D. L. Parker, C. Yuan, and D. D. Blatter, xe2x80x9cMR Angiography by Multiple Thin Slab 3d Acquisition,xe2x80x9d Magn. Reson. Med., Vol. 17, pp. 434-451, 1991), the volume of interest is scanned via the acquisition of a number of slabs, each acquired in a 3-D fashion while the different slabs are covered in a multi-slice fashion. In this case, the thin slabs allowed for good flow contrast while the 3-D encoding within the slabs provided good spatial resolution and an improved SNR. In spite of the success of such hybrid techniques, they are limited by their data inefficiency arising from slab overlapping (as much as 50%). Therefore, a technique that allows smooth and flexible combination of the characteristics of selective excitation and Fourier encoding while acquiring the data efficiently can be advantageous.
More recently, the application of MRI has been extended to interventional and dynamic imaging studies. In most of these applications, only a localized region needs to be updated rapidly. Such applications inspired the development of several novel encoding techniques such as wavelet encoding (See: D. M. Healy, Jr. and J. B. Weaver, xe2x80x9cTwo Applications of Wavelet Transforms in Magnetic Resonance Imaging,xe2x80x9d IEEE Trans. Info. Theory, Vol. 38, No. 2, pp. 840-860, 1992; and G. P. Zientara, L. P. Panych, and F. A. Jolesz, xe2x80x9cDynamically Adaptive MRI with Encoding by Singular Value Decomposition,xe2x80x9d Magn. Reson. Med., Vol. 32, pp. 268-274), and Singular Value Decomposition (SVD)-based encoding. Nevertheless, problems in the practical implementation and use of these techniques for actual clinical applications hindered such techniques from becoming realistic alternatives. Therefore, a spatial encoding technique that allows for fast localized image updating while keeping a simple implementation procedure is desirable for such applications.
The present invention provides a method and apparatus for spatial encoding using pseudo-Fourier imaging (PFI) that addresses the problem of magnetic resonance imaging with flexible excitation profiles acquired at a number of phase encoding steps. In particular, the invention provides an approach for spatial encoding based on acquiring a set of windowed Fourier transform coefficients. This new procedure embodying the invention is shown to be a general technique representing a flexible hybrid of selective excitation and Fourier encoding. In particular, the imaging technique of the invention corresponds to the multi-slice technique at one extreme and the Fourier encoding technique at the other. The condition under which the reconstruction process is stable are described demonstrating that its implementation can be readily achieved on the current MRI systems.